Atmospheric Sciences & Global Change Research Highlights

Decoding the Jet Stream Response to Global Warming

The jet stream is at its strongest when the contrast is greatest between cold Arctic air and warm tropical air that vie for dominance over weather patterns in the mid-latitudes. Climate warming forces are causing the jet to shift, and researchers are working to understand the dynamical forces that are at play. Graphics and animation are courtesy of NASA's Goddard Space Flight Center. Select this link to view the lively animation. Enlarge Image.

The
jet stream influences weather patterns over the northern mid-latitude region, a
place most of the world's people call home. Like an atmospheric "steering wheel," the jet stream directs cold fronts
and propels warm fronts eastward around the globe. The residents in the mid-latitudes
are familiar with typical weather events driven by the jet stream. Yet, like dropping
a powerful rock in the atmospheric "pond," climate forcing from
greenhouse gases can provoke the jet stream to shift in
unexpected ways.

Results: Led by Dr. Jian
Lu, an atmospheric scientist at Pacific Northwest National Laboratory,
researchers investigated changes in the jet stream using a series of climate simulations.
These simulations helped them to understand the forces causing shifts in the
jet that increase the survival of large-scale
weather events, technically known as atmospheric mid-latitude synoptic waves.
The surviving waves that peak about 12 miles above the Earth are in turn influenced
by forces at the poleward side of the jet that push more energy equatorward as the
jet drives eastward. The researchers investigated the impact of these vying forces
to understand how climate warming is affecting the seemingly erratic behavior
of the jet stream.

"This
study builds a bridge from an idealized, zonally symmetric world, to a realistic
3-D world," said Dr. Jian Lu, atmospheric scientist at
PNNL. "The study transfers our dynamical
understanding of the former to the latter."

Why It Matters: The mid-latitudes are where most of
the world's population lives. It's also where the jet stream influences weather
and the water cycle over most of the world's population. When polar air dipping
southward collides with rising warm tropical air, the meet-up causes a powerful
atmospheric wave with rollercoaster-like patterns that propagate eastward
around the globe.

For decades, atmospheric and climate scientists have puzzled
over how the jet stream is shifting, primarily in response to the increase of
greenhouse gases concentrating in the atmosphere. This study pinpoints the key
processes that are responsible for the dynamics of the jet shift. It also
builds a diagnostic framework with great potential for further development to
understand the linkage
between a local climate forcing and its global impacts. The jet stream is inextricably tied
to the global water cycle and the observed increase in extreme weather events,
motivating scientists to uncover the mysteries of jet stream activity.

Methods: In these
experiments, the research team conducted large ensembles of simulations with
two state-of-the-art atmospheric general circulation models by abruptly
switching the sea-surface temperature warming on from January 1st to focus on
the wintertime circulation adjustment. Performing a hybrid, finite-amplitude
wave activity budget analysis they elucidated the nonlinear and irreversible
aspects of wave interactions while the zonal wind adjusted toward a poleward-shifted
state. The experiments confirmed results from earlier more idealized studies,
particularly the importance of reduced wave dissipation activity, in which the
mid-latitude decrease of effective diffusivity appears to be dominant.

What's Next? The natural next step would be developing this dynamical diagnosis
toward three-dimensional, local wave activity to explain the dynamical
mechanisms at more regional scales. The
zonally asymmetric wave response in the subtropics and mid-latitude remains a mystery.
It is especially challenging with regard to the local/regional circulation
response to global warming. A local wave activity-centric diagnostic framework would
be a huge leg-up in tackling this challenging task.

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In short...

In one sentence: PNNL
researchers are uncovering how climate warming is affecting the seemingly
erratic behavior of the jet stream using a series of large-scale climate
simulations to understand the impact of vying forces on the jet.

In 100 characters: Researchers
uncover vying forces originating from climate warming to cause a shift in the
jet stream